Proper development and function of the anterior pituitary gland is critical for multiple physiological processes including growth, metabolism and reproduction. The mature anterior pituitary gonadotrope serves as a control center for the integration of hormonal signals in the regulation of reproduction. Gonadotrope development requires highly orchestrated transcriptional programs utilizing the coordinated activity of regulatory proteins to generate the terminally differentiated state; yet the mechanisms for gonadotrope maturation remain to be fully elucidated. The goal of this application is to investigate transcriptional regulation by homeodomain repressor Msx1 in gonadotrope differentiation and development. The decline of Msx1 expression coincides with the onset of expression of lineage-specific pituitary transcription factors 1GSU and GnRHR. My hypothesis is that Msx1 functions as a negative regulator in early pituitary development by repressing the early gonadotrope-specific genes: the common 1-subunit (1GSU) and GnRH receptor (GnRHR), thus coordinating the onset of the gonadotrope-specific gene program. Aim 1 will address the physiological relevance of Msx1 in 1GSU and GnRHR repression in the developing pituitary both in vitro and in vivo. Aim 2 will investigate the roles of Msx1 corepressors, the Transducin-Like Enhancer of split (TLE) proteins, their recruitment by Msx1, and the role of these recruitment events for 1GSU and GnRHR repression during gonadotrope differentiation. Finally, Aim 3 will delineate the functions of Msx1 and its cofactors in the maturation of the gonadotrope by epigenetic technologies. Overall, by utilizing our innovative cell lineage models, genomics, and genetically modified mice, these studies will provide novel insights into the molecular mechanisms of pituitary development and the onset of gonadotrope-specific gene programs.